1. Field of the Invention
The present invention relates to methods and apparatuses for sorting objects in DNA analysis. More particularly, the present invention relates to the sorting of sperm in a binding method using antibody or other sperm-recognizing biomolecules, and an apparatus thereof. In one embodiment, a polymerase chain reaction (PCR) method is performed after the objects are sorted—in some cases, a single cell PCR method—to identify persons/assailants in sexual assault cases in forensic DNA analysis, or for other applications in medical diagnostics.
2. Description of the Related Art
In conventional forensic DNA analysis, specimens are commonly matched to alleged criminal suspects in modern law enforcement, using human identification systems commonly based on short tandem repeats (STR) analysis which involve the amplification of the query DNA by polymerase chain reaction (PCR). PCR is a powerful tool which allows for replicating/amplifying trace amounts of DNA fragments into quantities that can be analyzed in a meaningful way. This technology has been adapted for DNA sequencing, DNA fingerprinting etc., and has the ability to detect specific DNA fragments in samples.
Thus, forensic DNA analysis is accomplished using the high power of discrimination and rapid analysis speed of STR markers in the human genome, and has now become the most popular method of choice in forensic DNA analysis.
Although STR analysis is commonly used, it suffers from several pitfalls, the most significant of which arises from contamination of the DNA samples prior to PCR (Polymerase Chain Reaction) based STR analysis, and the time it takes to perform the entire STR analysis on a given sample.
For example, the DNA to be analyzed for STRs from sexual assault evidence should ideally come from the sperm of the assailant. However, the sperm sample is often commonly contaminated with (1) epithelial cells lining the vagina, and occasionally, with (2) epithelial cells from the mouth (buccal cells), and (3) cells from the skin, as well as cells in the urine sample. One might also expect to see erythrocytes, neutrophils, foam cells (non-descript epithelial cells), etc., in sexual assault crime scene samples as well.
Thus, it is clear that better and more accurate STR analysis will be achieved if the sperm cells could be separated from any or all of the contaminating cells before PCR is performed.
Commonly used methods of differential extraction cannot completely separate male (assailant) sperm and female (victim) epithelial cell DNA in a forensic sample. For example, initial lysis using reductant free solution, lyses epithelial cells (the most common contaminant in a sexual assault forensic sample), and leaves sperm cells intact for effective separation of DNA fractions. However, differential lysis causes immature sperm cell lysing thereby causing unwanted DNA to be coamplified along with the query DNA (from sperm alone). This leads to mixed STR profile generation which are hard to analyze and cannot identify a unique individual. Such problems in STR analysis causes 50% of the STR analysis based human identification to fail.
In addition, another limitation in solving forensic cases comes from the limited availability of cells for analysis. This may be due to limited evidence samples being present, degradation of the DNA and cell samples in general over time, and/or the presence of very few sperm cells in a sexual assault crime sample, to be able to solve the case based on standard PCR.
Thus, a method that would prevent or alleviate the above problems is desired.